1. Field of the Invention
The present invention relates to a semiconductor device and a method for fabricating the same. In particular, the present invention relates to a semiconductor device using thin film transistors formed on a glass or other insulating substrates and to a method for fabricating the same. More specifically, the present invention relates to a semiconductor substrate having a crystalline silicon film crystallized from an amorphous silicon film formed on a substrate having an insulating surface, to a semiconductor device utilizing the thus obtained crystalline silicon film as an active region, and to a method for fabricating such a semiconductor substrate and a semiconductor device. In particular, such a method is applicable to fabrication of an active matrix type liquid crystal display device, an image sensor, a three-dimensional IC and the like.
2. Description of the Related Art
Recently, it has been attempted to form semiconductor elements having good performance on insulating substrates, made of materials such as glass, or on insulating films which are formed on a surface of a substrate, for realizing large liquid crystal display devices having a high resolution, close-contact type image sensors having a high resolution with high speed, or three-dimensional ICs and the like. In general, thin film transistors (TFTs) are used for driving pixels in active matrix type liquid crystal display devices, image sensors and the like. Generally, the TFTs used in these apparatuses are formed from silicon semiconductor thin films.
Such silicon semiconductor thin films are roughly classified into two types: amorphous silicon (a-Si) semiconductor films and crystalline silicon semiconductor films.
Of these two types, the amorphous silicon semiconductor film is preferred and enjoys general uses because it has a low processing temperature and is easily manufactured using a vapor deposition method, thus lending itself to mass production. Compared to the crystalline silicon semiconductor film, however, the amorphous silicon semiconductor film is inferior in properties such as electrical conductivity. It is therefore strongly desired to establish an efficient fabrication method for TFTs formed from the crystalline silicon semiconductor films to achieve faster response characteristics of the semiconductor devices fabricated from them.
The crystalline silicon semiconductor films currently known include polycrystalline silicon, micro-crystalline silicon, amorphous silicon containing crystalline components, semi-amorphous silicon having an intermediate state between crystalline and amorphous forms, etc. The following three methods are known for the production of these thin film crystalline silicon semiconductors.
(1) A first method in which a crystalline silicon semiconductor film is directly formed in a film deposition step.
(2) A second method in which an amorphous semiconductor film is first formed, followed by laser radiation to crystallize the amorphous silicon film by the laser's optical energy.
(3) A third method in which an amorphous semiconductor film is first formed, followed by application of heat energy to crystallize the amorphous silicon film.
However, the above methods have the following disadvantages.
According to the first method, since the crystallization proceeds during the deposition step, a thick silicon film must be formed to obtain a crystalline silicon film with a large grain size. Consequently, it is technically difficult to form a film having good semiconductor properties uniformly over the entire surface of the substrate. Furthermore, since the film needs to be deposited at high temperatures of 600.degree. C. or more, this introduces disadvantages in cost in that inexpensive glass substrates cannot be used since they have low softening temperatures.
The second method utilizes the crystallization in the melting and solidification processes, and allows the formation of a high-quality silicon film with a small grain size and yet having properly treated grain boundaries. However, with the lasers commonly used today, such as the excimer lasers for example, the processing throughput is low because the effective laser beam radiation area is small. A further disadvantage is that the stability of lasers is not sufficient to uniformly process over the entire surface of a large substrate.
The third method, which crystallizes silicon in solid phase by using heat energy, has an advantage over the first and second methods in that a thin-film crystalline silicon film can be formed uniformly over a large substrate. Examples of this method are disclosed in Japanese Laid-Open Patent Publication Nos. 62-122172, 3-290924, and 4-165613. According to the methods disclosed therein, however, all of the methods require a heat treatment at high temperatures of 600.degree. C. or more for several tens of hours to accomplish crystallization. Therefore, to allow the use of inexpensive glass substrates and to increase the processing throughput, contradicting requirements need to be satisfied simultaneously, i.e, to lower the processing temperature and to accomplish crystallization in a short period of time. Furthermore, since all of these methods utilize the solid-phase crystallization, crystal grains spread in parallel to the substrate surface and crystals having a grain size of a few micrometers may be formed. During this crystal growth process, grain boundaries are formed while the growing crystal grains are colliding with one another, resulting in the grain boundaries having lattice defects. Consequently, the grain boundaries act as carrier traps and as a result, a carrier mobility in the resultant TFTs decreases.
Thus, in order to solve the problems in the crystal grain boundaries utilizing the third method described above, the following fourth and fifth methods have been proposed.
In the fourth method, impurity ions such as silicon (Si.sup.+) are introduced into the amorphous silicon film by ion implantation or the like and, by a subsequent heat treatment, a polycrystalline silicon film having crystal grains a size of about a few micrometers is formed (Japanese Laid-Open Patent Publication No. 5-55142). In the fifth method, silicon grains of sizes from 10 to 100 nm are sprayed over the amorphous silicon film with pressurized nitrogen gas, thus forming the crystal nuclei (Japanese Laid-Open Patent Publication No. 5-136048).
In both methods, after selectively introducing foreign substances serving as nuclei of crystal growth in a predetermined region of the amorphous silicon film, crystal growth proceeds using the foreign substances as nuclei by heat treatment, thereby obtaining a crystalline silicon film of a large grain size. Then, a semiconductor device such as a TFT is formed by utilizing the obtained crystalline silicon film.
According to the fourth and fifth methods, introduced foreign substances act only as nuclei for crystal growth. Thus, the methods are effective for generating the nuclei during the crystal growth and controlling the direction of crystal growth. However, the above described problems in a heat treatment process for crystallization still remain. For example, in the fourth method, crystallization is performed by heat treatment at a temperature of 600.degree. C. for 40 hours. In the fifth method, a heat treatment is performed at a temperature of 650.degree. C. or more. Since the heating is performed at a high temperature as described above, these techniques are effective for a SOI (Semiconductor on Insulator) substrate or a SOS (Semiconductor on Sapphire) substrate. However, it is difficult to apply these techniques to low-cost glass substrates. For example, the distortion temperature for Corning 7059 glass, which is commonly used in active-matrix liquid-crystal display devices, is 593.degree. C. Thus, in the case where a liquid crystal display device having a large area is fabricated using this particular substrate by the above method, the substrate is likely to be deformed by heating at temperature of 600.degree. C. or more.
Furthermore, in connection with the above third method, that is, the method for crystallizing an amorphous silicon film utilizing heating treatment, Japanese Laid-Open Patent Publication No. 6-244205 published on Sep. 2, 1994 and Japanese Laid-Open Patent Publication No. 6-260651 published on Sep. 16, 1994 each discloses that the crystallization process can be realized at a low temperature for a short period of time by introducing heavy metal elements such as nickel, iron, cobalt and platinum into the amorphous silicon film as catalyst elements. Moreover, Japanese Laid-Open Patent Publication No. 6-268212 published on Sep. 22, 1994 teaches the optimum minimum concentration of the catalyst elements in the silicon film in connection with the above methods.
However, there still remain the problems to be solved in order to apply the methods disclosed above to formation of the crystalline silicon film over the large substrate and fabrication of the semiconductor device using the resultant semiconductor substrate.